1. Field of the Invention
The present invention relates to a thermoplastic molding composition having high impact strength and enhanced oxidation stability based on a physical blend of polystyrene with a hydrogenated thermoplastic elastomeric block copolymer and low density polyethylene, together with a process for preparing such a composition by physically blending together such components.
2. Description of the Prior Art
High impact molding compositions based on polystyrene conventionally contain polymeric styrene and a rubber polymer. It is known that good compositions may be obtained by physical blending of polystyrene with suitable rubbers, or by mixing the rubber polymer with monomeric styrene and graft polymerizing the mixture. Many types of rubber materials have been employed including natural rubber, polybutadiene, and styrene/diene copolymers. Such polystyrene compositions, although having acceptably high impact strength, do not normally have good oxidation stability, which causes them to deteriorate rapidly with exposure to sunlight and thereby suffer a reduction in their impact strength.
A valuable improvement in oxidative stability can be achieved through the physical blending of polystyrene with hydrogenated thermoplastic rubbers as described in U.S. Pat. No. 3,810,957, namely, with a block copolymer prepared by selectively hydrogenating the diene portion of a block copolymer having at least two polymer blocks of a monovinyl arene separated by at least one polymer block of a conjugated diene. However, in order to be commercially acceptable it is, of course, necessary for a product not only to have satisfactory properties but also to be capable of being marketed at a commercially acceptable price. The hydrogenated block copolymers utilized in the composition blends of U.S. Pat. No. 3,810,957 increase the overall cost of the final blended product, and it would therefore be desirable if alternative composition ingredients could be found which would enable the desired properties to be attained with a reduced level of block copolymer.
The applicant has discovered that this desirable objective can be met by substituting low density polyethylene for part of the block copolymer used in the compositions of U.S. Pat. No. 3,810,957. Compositions suitable for expansion by a blowing agent are disclosed in Belgian Pat. No. 826,831, said compositions comprising polystyrene, polyethylene and a mixing adjuvant/compatabilizing agent. The general disclosure of that document describes a very broad range of polymer blends, including polystyrene/low density polyethylene/hydrogenated styrene/butadiene copolymers, but the specific examples and detailed explanations clearly indicate that the products actually envisaged contain substantial amounts (sometimes even a major amount) of polyethylene. No specific disclosure is made of compositions containing a substantial predominence of polystyrene, nor is there any suggestion in that disclosure that the blending of minor amounts of low density polyethylene and hydrogenated block copolymers with polystyrene will yield a high impact polystyrene composition having good oxidation resistance.